One of the imperative with a circuit breaker is handling high overload and short circuit currents which involves arc extinguishing features. Shield arrangements have been designed for rapidly extinguishing arcs in circuit breakers with a high current interrupting capability. There is, however, another requirement which is to maintain minimal size requirements and low cost construction. The latter, as shown in U.S. Pat. No. 3,566,318, has been solved in the past with a housing composed of electrically insulating material, such as a thermosetting resin, within which all the sophistication of an effective full capability circuit breaker has been accommodated. One of the major features, so incorporated in a relatively small housing, has been to provide an arc chamber, centered in the plane of the trajectory of the movable contact pulled away from the stationary contact, wherein the arc can stretch out until it breaks. As shown for an AC circuit breaker by U.S. Pat. No. 4,266,210, an improvement thereon has been to provide an U-shaped member of ferromagnetic material (such as steel) aligned with the sidewalls of the housing and defining an inner channel wherein the arc is pulled away and split into smaller arcs by the eddy-currents induced in the U-shaped member. This works only with AC current.
It is also known for DC circuit breakers to use a magnet applying a field to the arc chamber in a direction such as to force the arc away from its shortest trajectory when the movable contact is pulled away from the stationary contact, thereby to help break the arc and allow protection against higher DC currents.
It is desirable to avoid redesigning a circuit breaker for DC usage when it is already available and known on the market for AC applications usage, and conversely. A model which has all the qualities desired in terms of effectiveness, cost, size, insulation, and as available on the market, should preferably be compatible for AC and DC applications in a more general use. Therefore, a circuit breaker has now been designed which can be used for those fields of application and, while so doing, minimal changes have been called for without impairing the commercial quality of the circuit breaker as it is known for either of these two fields of application.